[kag] Re: GPS accuracy?

  • From: Hoover <mlehotay@xxxxxxx>
  • To: kag@xxxxxxxxxxxxx
  • Date: Mon, 16 Aug 2004 12:28:42 -0400 (EDT)

On Mon, 16 Aug 2004, Gordon Campbell wrote:
> Does anyone know how reliable the speed reading on a GPS is? My sTrex
> consistently insists that my car is travelling at exactly 5km/hr less
> than the speedometer. hmmmm.

I saw the same thing with Root's old car. I assumed it was a calibration 
issue with the speedometer. I should check again in the new car. Now that 
I think of it, this is more likely related to the GPS receiver not 
including changes in elevation in its calculations. I think it projects 
your position onto its mathematical model of the Earth and measures your 
movement along that. So if you are driving up a hill, your projected 
motion would be slower than what your speedometer says. Or maybe the model 
is not quite parallel to the surface of the Earth in our part of the 
world. I'm just guessing at all of this, of course.


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